Disposable articles such as diapers, adult incontinence products and feminine hygiene pads are generally constructed of three major components: a top sheet, an outer backsheet, and an intermediate liquid absorbent core. In normal use, the top sheet contacts a user's skin and provides channels to transport body fluids to the absorbent core which functions to retain the body fluids. In order to keep the fluids within the absorptive devices, typical backsheets are formed of leakproof materials. Recent diaper constructions incorporate elastic waist and leg strips to close the openings around the waist and the legs to prevent fluid leakage. Such designs, however, minimize air flow through the leg or waist openings and, since a typical backsheet is vapor impermeable, the interior of the diaper tends to become very damp and warm due to inadequate ventilation. This can result in diaper rash or other types of skin irritation. In addition, typical backsheets also rattle and crinkle, which can be of major concern to users of adult incontinence and feminine hygiene products. The noise produced is generally due to the stiffness of the backsheet. The other two components, the top sheet which is typically a soft perforated film or a light-weight nonwoven web (U.S. Pat. Nos. 3,044,467, 3,643,154) and the absorbent core which is mainly fluff pulp and tissue paper (U.S. Pat. No. 3,881,490) or high absorbency fibers (U.S. Pat. No. 3,570,491), do not contribute in any significant manner to the noise generated when the articles are twisted or rubbed.
Prior workers have attempted to produce rattle-free disposable products. U.S. Pat. No. 4,059,114 describes the use of a soft, rattle-free, vapor-permeable, body fluid impermeable web formed of blown microfibers in a feminine hygiene garment shield. A known disadvantage of blown microfiber material is that it can have an insufficient water holdout strength which results in liquid strike-through. The patent is also directed to the use of an additional moisture barrier formed of a butadiene-styrene copolymer, which is soft, rattle free, and high in water hold out but is not vapor permeable.
EPA 0,194,150 discloses the use of an elastomeric diaper backsheet formed of either ethylene-propylene-diene (EPDM) or ethylene propylene elastomer (EPM), mixed with ethylene-vinyl acetate copolymer (EVA) and an aromatic hydrocarbon oil. This approach provides low noise and good hand characteristics for diapers but offers no air permeability (breathability).
The art is replete with a wide variety of methods for producing porous films for disposable articles. U.S. Pat. No. 3,426,754 discloses a method for producing a microporous film for use as a leakproof backsheet for items such as diapers, sanitary napkins, and medical dressings. Microporosity is achieved by uniaxially cold stretching a polyolefin film until a void structure is formed by film failure, subsequently hot stretching the cold stretched film until the voids become open cells and finally heat setting the open structure. Controlled porosity is generally not attained and such a film tends to split when stretched in a direction transverse to that of the orientation.
U.S. Pat. No. 4,341,216 discloses a disposable diaper with a vapor permeable, relatively liquid impervious backsheet. The backsheets of the patent do not have the reduced rattle properties of films of the present invention.
Microporous sheet materials can also be made by incorporating a soluble filler in the sheet and then dissolving the filler to provide pores of various sizes within the material as described in U.S. Pat. Nos. 3,214,501 and 3,640,829. U.S. Pat. Nos. 3,844,865 and 3,870,593 disclose a process for making a porous structured polymeric film by dispersing particles which can be left in the film. Particles of inorganic salts such as calcium carbonate are blended into a polymer; a film is formed of the filled polymer and subsequently the film is stretched to provide porosity. The calcium carbonate fillers which remain in the final film serve as pore inducing agents when the film is stretched. While the porous films described, such as polypropylene, exhibit good strength, they have only limited moisture vapor transmission rates and the films are not rattle-free.
U.S. Pat. No. 4,626,252 describes a porous film for use as the leakproof sheet of a disposable diaper. The porous films are produced by blending a polyolefin resin with a filler and a liquid or waxy hydrocarbon polymer, forming the resulting resin composition into a film, and then stretching this film at least uniaxially by a factor of 1.2 or greater. The fillers act as pore inducers and the hydrocarbon polymer functions as both a processing agent and a plasticizer. As in U.S. Pat. Nos. 3,844,865 and 3,870,593, the porosity can be increased by increasing the amount of filler in the film and by increasing the stretch ratio. However, when the amount of filler is increased to obtain higher porosity, the strength of the film decreases and there is a limit on the amount of stretch attainable. As a result, a limitation of this approach is its inability to obtain high permeability while maintaining satisfactory strength. In addition, pore uniformity depends on particle uniformity and dispersion in the film. The high percentage of filler and the absence of a rattle-reducing additive produces a film which does not have the rattle-free properties of the films of the present invention.
U.S. Pat. No. 4,699,733 describes a soft, vapor permeable film for use as the leakproof backsheet of a disposable diaper. The film is produced by blending a polyolefin resin and barium sulfate fillers, forming the molten resin composition into a film, and then stretching the film at least uniaxially by a factor of 1.5 to 7. The barium sulfate fillers act as the pore inducers at the time of stretching. This method has the same disadvantages as that associated with U S. Pat. No. 4,626,252. It does not achieve a good balance of permeability and strength.
U.S. Pat. No. 4,539,256 discloses another approach for making microporous films. A crystallizable thermoplastic polymer is melt blended with a compound in which the polymer will phase separate upon cooling below the crystallization temperature of the polymer. The mix is extruded in sheet form, followed by orientation in at least one direction, resulting in a microporous sheet with micropores distributed therethrough. The compound can then be removed from the microporous sheet by extraction, evaporation, leaching and the like to achieve high permeability.
U.S. Pat. No. 4,609,584 discloses an embossable porous film with satisfactory strength and high permeability for use as a diaper backsheet. At levels of high permeability, this film is also noisy which makes it less desirable for use in disposable products such as diapers and feminine hygiene products.
Copending U.S. application Ser. No. 940,731 discloses a method for making microporous films by incorporating a nucleating agent in a melt blend of a thermoplastic polymer and a compound miscible at the melting temperature of the polymer and which phase separates upon cooling at or below the crystallization temperature of the thermoplastic polymer. The incorporation of a nucleating agent makes the films stronger and more permeable than equivalent films without a nucleating agent.